With the rapid development of broadband services that has accompanied the spread of fiber-optic communications in recent years, communication capacity has been increasing yearly. The increased capacity of optical networks handling the rapid increase in communication capacity has, until now, been achieved by increasing the capacity of optical communication system apparatus without changing the structure of the optical fiber. The optical fibers that form the basis for current high-capacity optical networks are single-mode fibers. Single-mode fibers are optical fibers in which there is a single core that serves as the passage for optical signals in each optical fiber, and which are excited in only a single mode. These are able to form optical networks that have the capacity to transmit several terabits per second over long distances.
However, due to the rate of increase in communication capacity in recent years, a pressing issue has been the manner in which the communication capacity can be further increased. By using, as the transmission medium, an optical fiber, known as a multi-mode fiber, which has a core through which signal light propagates in multiple modes, the communication capacity per fiber can be increased by the number of modes that are used. However, multi-mode fibers have higher transmission loss and the like compared with single-mode fibers, and signals that have undergone intermodal coupling during propagation must be separated after being received. For this reason, multi-mode fibers have been considered to be unsuitable for long-distance transmission, and have been used only in limited fashion, mainly as transmission media for short-distance communications such as in LANs (Local Area Networks). However, due to the development of MIMO (Multiple-Input Multiple-Output) signal processing technology used in wireless communication systems and optical amplifiers for multi-mode fibers in recent years, there has been active research and development into mode-division multiplexing optical communication schemes using multi-mode fibers, and the possibility of a solution to the above-mentioned problem of increasing the communication capacity has been suggested.
In order to put into practice the above-mentioned mode-division multiplexing optical communication scheme using multi-mode fibers, which has been actively considered in recent years, it is essential to have a technology that makes it possible to stably transmit optical signals over long distances without lowering the quality thereof. Performance evaluation indicators for multi-mode fiber transmission include intermodal dispersion and mode-dependent loss, and it is important to control and compensate the optical signals on the basis of these indicators so that the quality is not lowered. Of the above-mentioned indicators, mode-dependent loss is a phenomenon whereby the optical intensity loss differs for each propagation mode, which occurs, for example, in multi-mode optical amplifiers and mode multiplexer/demultiplexers. The phenomenon that occurs in multi-mode optical amplifiers is, to be precise, mode-dependent gain, but in the following description, mode-dependent loss will be used as an expression that includes mode-dependent gain.
However, in conventional multi-mode fiber transmission, there is significant degradation of optical signals due to mode-dependent loss, thus precluding longer-distance multi-mode fiber transmission. One might contemplate using, for example, a multi-span optical transmission system in which multi-mode fibers and multi-mode optical amplifiers are arranged in multiple stages. While multi-mode optical amplifiers are used for the purpose of compensating for transmission loss that has occurred on a multi-mode fiber, mode-dependent loss generally occurs in multi-mode optical amplifiers, so it is not possible to precisely compensate for the transmission loss of all of the mode signals. The mode-dependent loss that occurs on each span accumulates as a signal passes through the spans, and ultimately, it becomes impossible to transmit specific mode signals. As another example, one might contemplate transmitting a single data sequence by multi-mode fiber transmission. Due to the mode-dependent loss that occurs during transmission, the signal quality of specific modes will be degraded, and under the influence thereof, the optical signal quality of all of the modes will, on average, be largely degraded, as a result of which the transmission of the single data sequence may be greatly affected. If the mode-dependent loss that occurs on the transmission path is known at the transmitting end, a transmission method with good transmission efficiency that takes the mode-dependent loss into consideration could be provided, for example, by using the water-filling principle or the like. However, it is known that mode-dependent loss generally varies probabilistically in association with mode coupling (see, e.g., Non-Patent Document 1), and it is difficult to continuously pre-acquire mode-dependent loss information at the transmitting end by means of feedback control from the receiving end.